Mechanized coal preparation plants use large quantities of water to separate fine particles of coal and other material from the graded coal aggregates. The underflow from the wash treatment typically contains 5 to 15% solids of coal and ash having particle sizes in the range less than 150 microns (i.e. 100 mesh Tyler sieve). Typically, the underflow is flocculated by addition of, for example, starch or polyacrylamide in a thickener. The solids content of the underflow is increased to typically about 50% in the thickener, and water reclaimed for recirculation to the coal washing plant.
The thickened underflow is then generally pumped to a settling pond (or "lagoon") where the solids settle out and the reclaimed water is made available for reuse in the coal washer. Such settling ponds are, however, increasingly difficult to construct and maintain. There is often limited land available at the coal washing plant, which requires pumping of the underflow over long distances at substantial cost. Moreover, even where there is land available adjacent the washing plant, environmental laws have made such settling ponds difficult and expensive to construct and operate in compliance with state and federal regulations. And, permanent use of settling ponds presents the problems and costs of disposing of the sediment.
Various processes and apparatus have been proposed to eliminate the use of settling pond, i.e. filter presses, vacuum filters, centrifuges, and aqua-pelletizers as described in U.S. Pat. Nos. 3,606,947 and 3,630,893. The main disadvantages of these techniques are restricted and variable capacity, high capital and operating costs and high manpower requirements. Almost invariably these techniques still are preceded by a conventional thickener to increase the solids concentrations of the feed. Further, a filter press is typically a batch-wise process whose cycle can vary widely, e.g., 1 to 4 hours; a conventional vacuum filter has unique difficulties in separating the fine particles present in the tailings; and a commercial centrifuge readily ruptures and breaks-down conventional flocs requiring the use of expensive shear-resistant flocculants. Finally, the aqua pelletizer is a specialized expensive piece of equipment that requires a trained operator.
Another more efficient process and apparatus has been the cone thickener, which can obviate the need for a preceeding thickening operation as well as succeeding dewaterizing operation. The cone thickener is generally cylindo-conical in shape with considerably less settling area and considerably more depth than the conventional thickener. The feed premixed with flocculant is charged to the center top of the thickener. The solids settle through the thickener aided by gentle stirring to consolidate the flocs, and form a plug at the bottom apex of the cone. Clarified water overflows the periphery about the top of thickener and is reclaimed for use in the coal washing plant.
The main problem with the use of the cone thickener is the wide variation in the nature, size and percentage of solids content of the underflow even though the coal may originate from the same seam. This problem is compounded by recent processes that extract coal fines from the underflow by froth flotation or hydrocarbon beneficiation, see U.S. Pat. No. 3,665,066. These processes reclaim the coal fines leaving the tailings with a solids content generally of only 1 to 5%. Although methods are available for automatic control of the feed and variation of the flocculant additional relative to the solids content thereof, such systems do not satisfactorily moderate the variations in solids concentration of the tailings.
Maximum solids pressure is essential to obtain discharge of high solids concentrations. A cone is therefore typically operated at or near the maximum depth of thickened solids compatible with clear water from the overflow. Ideally, the solids content of the discharge is maintained substantially constant, with the rate of discharge controlled by the density of the solids concentration at the discharge; but solids concentrations at the discharge was not heretofore be directly measured and controlled. Rather, generally the differential pressure vertically in the cone due to the suspended solids is measured, and on reaching a given differential pressure, the pneumatically controlled valve at the bottom of the cone is automatically opened until the pressure drops below a given value when the valve is automatically closed. This system is sensitive to changes in solids concentration and therefore has not been satisfactory. An alternative proposal has been to continuously extract a sample from near the discharge point of the cone and pump the sample through specific gravity measuring device, insensitive to fluctations in flow rate and pressure, returning the sample to the cone at a higher level-- the specific gravity measurement being used to continuously control the discharge through the valve at the apex of the cone. This alternative is slow in response so that there is a time lag and in turn a variation in the solid concentrations of the discharge.
The present invention overcomes these disadvantages and difficulties by directly measuring the density of the solid concentrations of the discharge and controlling the rate of discharge without any appreciable time lag. The solids content of the discharge is in turn improved and more constant. The present invention also permits direct control of the solid concentrations discharged from supplemental dewatering devices utilizable with thickeners, such as device described in applicant's U.S. Pat. No. 3,423,313, as well as a simple, inexpensive method of further controlled dewaterizing on discharge from the thickener.